Correct functioning of the visual system requires neural connections from eye to brain to develop with precisely defined spatial order. In addition to following correct pathways, connections must be arranged in topographic maps: maintaining neighbor relationships so that spatially accurate images of the visual world are formed in the brain. Topographic maps are believed to develop in response to complementary gradients of positional labels in presynaptic and postsynaptic fields, an idea based mainly on more than half a century of research on the visual retinotectal projection. In recent years, a new family of cell-cell signaling molecules, the ephrins, has been identified. Ephrins have topographically specific guidance effects on retinal axons, and are required for orderly development of axon projections from the retina to the brain. While specific mechanisms, and even broad principles, of visual map specification remain poorly characterized, the identification of ephrins as topographically specific labels allows a new level of molecular investigation in this field. The broad goal of this proposal is to investigate molecular mechanisms in development of visual neural connections. The major focus is on ephrins and their receptors in axon guidance and mapping in the retinotectal system. Aims and methods include: (1) Analysis of retinal axon responses to ephrins, or tectal membranes, especially by new in vitro assays, (2) Action and modulation of ephrins as labels in visual map development in vivo, including transgenic and new small-molecule approaches for directed restructuring of connections, (3) Mechanisms that convert ephrin signaling to retinal axon responses, (4) Other cues with instructive or permissive roles in mapping, particularly protein tyrosine phosphatases and their ligands. While visual maps have historically been the best studied, topographic mapping is a major organizational principle throughout the nervous system. The studies here are expected to contribute to the basic understanding of map development at the molecular level. This should also help in providing a basis to understand abnormalities that lead to disease. Ultimately, studies to identify, characterize, and modulate novel cell-cell signaling molecules, may lead to therapeutic agents for maintenance, repair or regeneration of visual connections.